Question:

What is a brown dwarf?

Answer:

In order to understand what is a brown dwarf, we need to understand
the difference between a star and a planet. It is not easy to tell a star
from a planet when you look up at the night sky with your eyes. However, the
two kinds of objects look very different
to an astronomer using a telescope or spectroscope. Planets shine by
reflected light; stars shine by producing their own light. So what makes
some objects shine by themselves and other objects only reflect the light
of some other body? That is the important difference to understand -- and it
will allow us to understand brown dwarfs as well.

As a star forms from a cloud of contracting gas, the temperature in
its center becomes so large that hydrogen begins to fuse into helium --
releasing an enormous amount of energy which causes the star to begin
shining under its own power. A planet forms from small particles of dust
left over from the formation of a star. These particles collide and stick
together. There is never enough temperature to cause particles
to fuse and release energy. In other words, a planet is not hot enough
or heavy enough to produce its own light.

Brown dwarfs are objects which have a size between that of a giant
planet like Jupiter and that of a small star. In fact, most astronomers
would classify any object with between 15 times the mass of Jupiter and 75
times the mass of Jupiter to be a brown dwarf. Given that range of masses,
the object would not have been able to sustain the fusion of hydrogen like
a regular star; thus, many scientists have dubbed brown dwarfs as "failed
stars".

Starting in 1995, astronomers have been able to detect a few nearby
brown dwarfs. All of the brown dwarfs discovered so far are parts of a
binary system. A binary system is one in which two stars orbit around one
another (just like the planets of our solar system orbit our star, the
Sun).

So why would we care about brown dwarfs? It is possible that a great
deal of the mass in the universe is in the form of brown dwarfs, and since
they do not give off much light, they could constitute part of the "missing
mass" problem faced by cosmology.